The present invention pertains to suction devices for feeding sheets in sheet-separating or sheet-conveying apparatus, employed, for example in printing machines and operated in accordance with a pressure differential principle.
Known suction devices of this type can in principle differ from each other. For example, suction devices have been known, in which the initial position of the suction piston exactly corresponds to the position, which the suction piston takes, after the suction of a sheet thereby has been completed.
Due to the fact that the suction devices of this type are simple in construction and functionally reliable they have been widely employed in a sheet-feeding equipment. The structure and the function of such a suction device has been disclosed, for example in DD-PS No. 106,018.
Conventional suction devices of the foregoing type have the disadvantage that the suction piston, after the sheet has been sucked by a suction nozzle connected to the suction piston, that is at the end of the backward stroke of the piston in the direction of the housing end wall, is accelerated and strikes against this wall. This takes place due to a pneumatic pressure force, the value of which is determined by a pressure difference between the atmospheric pressure and under-pressure in the suction device and by the cross-section of the piston rod and also by the action of the force of the compression spring positioned in the suction device. An impact energy of the suction piston is therefore dependent upon a pressure force, a spring force, a piston mass and a piston stroke and increases with the stroke length increase. This takes place, particularly with greater suction strokes of the suction piston, which strokes are required during the separating the sheets from the stack having a buckled upper surface. The impact action of the suction piston due to its acceleration also leads to higher loading on the suction piston and the housing so that these structural components can be eventually damaged.
Furthermore, the impact occurring in the suction device is transmitted to the sheet-separating and sheet-conveying mechanisms and leads to an overloading and thus to an extensive wear of these mechanisms. The precision of the operation of the sheet-separating and conveying devices is negatively affected because the impacts occurring in these mechanisms cause vibrations. To avoid at least some of the above described negative effects it has been suggested until now to make the housing and the sheet-separating and sheet-conveying mechanisms of greater dimensions, which of course has involved a higher material consumption and also caused greater mass forces in the mechanisms of the sheet-separating device so that its ability to take loads has been limited.